OA11363A - Rotating electric machine. - Google Patents

Abstract

A rotating electric machine of a type with rotating field circuit, intended for direct connection to a distribution or transmission network. At least one electric winding of the machine comprises at least one electric conductor, a first layer with semiconducting properties surrounding the conductor, a solid insulating layer surrounding the first layer, and a second layer with semiconducting properties surrounding the insulating layer. A detecting circuit (16) is also arranged to detect earth faults in the rotating field circuit. Methods of monitoring the resistance of the field winding to earth and of determining the rotor temperature in such a machine is also described.

Description

011363

ROTATING ELECTRIC MACHINE

Technical field

The présent invention relates to a rotating eiectric machine of s type wit'n5 rotating field circuit, v/hich machine is intended for direct connection to adistribution or transmission network! The invention also relates to t'ne method ofmonitoring the résistance of the field winding to earth and of determining the rotortempérature. 10 Backaround art

The rotating eiectric machine according to the présent invention may be e.g. a synchronous machine, dual-fed machine, asynchronous static currentconverter cascade, external pôle machine or synchronous fiow machine.

In order to connect machines of this type to distribution or transmission15 networks, in the followed called power networks, transformers hâve previouslybeen used to step ud the voltage to the level of the network, i.e. to the range of130-400 kV.

Generators having a rated voltage of up to 3S kV are described by Paul R.Siedler in an article entitled “36 kV Generators Arise from Insulation Research",20 Electrica! World, 15 October 1932, pages 524-527. These generators comprisewindings of high-voltage cable in which the insulation is divided into various layershaving different dielectric constants. The insulatina matériel used consists ofvarious combinations of the three components mica-foii-mica, varnish and paper.

It has now been discovered that by manufacturing windings for the25 machine mentioned in the introduction out of an insulated high-voltage eiectricconauctor wifn soiid insulation of a type simiiar to cables for power transmission,the voltage of the machine can be increased to such leveis that the machine canbe connecter directiy to any power network wit'nout an intermediate transformer.A typical operatina range for these machines is 30 to 800 kV. 30 Furthermore, in system solutions based on brushiess'· excitera for excitation of a synchronous machine, for instance, the rotor winding of thesynchronous machine is normally not monitored for earth faults. 01 1 363

The object of the présent invention is to provide such a rotating electricmachine for direct connection to power networks, with the ability to detect earthfaults in the rotating field circuit. 5 Summarv of the invention

This object is achieved with a rotating electric machine of the typedescribed in the introductory portion with the characterizing features defined inclaim 1.

The insulating conductor or high-voltage cable used in the présent10 invention is flexible and is of the type described in more detail in WO 97/45919and WO 97/45847. The insulated conductor or cable is described further in WO 97/45918, WO 97/45930 and WO 97/45931.

Thus, in the aevice in accordance with the invention the windings arepreferably of a type corresponding to cables having solid, extruded insulation, like 15 those currently used for power distribution, such as XPLE-cables or cables withEPR-insulation. Such a cable comprises an inner conductor composed of one ormore strands, an inner semiconducting layer surrounding the conductor, a solidinsulating layer surrounding this inner semiconducting layer and an outersemiconducting layer surrounding the insulating layer. Such cables are flexible, 20 which is an important property in this context since the technoloay for the machineaccording to the invention is based primariiy on winding Systems in which thewinding is formed from conductors which are bent during assembly. The fiexibilityof a XPLE-cable normally corresponds to a radius of curvature of approximately20 cm for a cable 30 mm in diameter, and a radius of curvature of approximately 2; 65 cm for a cabie 80 mm in diameter. In the présent application the term “flexible” is used to indicate that the winding is flexible down to a radius cf curvature in theorder of four times the cable diameter, preferably eiaht to twelve times the cablediameter.

The winding should be constructed to retain its propenies even when it isbent and when it is subjected to thermal or mechanical stress during'operation. Itis vital that the layers retain their adhesion to each other in this context. Thematerial properties of the layers are décisive here, particularly their elasticity and 011363 relative coefficients of thermal expansion. In a XPLE-cable, for instance, the insulating layer consists of cross-iinked, iow-aensity poiyet'nylene, and thé semiconducting layers consist of polyethylene with soot and métal particles mixed in. Changes in volume as a resuit of température fluctuations are completely 5 absorbed as changes in radius of the cable and, thanks to the comparativelyslight différence between the coefficients of thermal expansion in the layers inrelation to the elasticity of these matériels, the radial expansion can take placewithout the adhesion between the layers being lost.

The materiai combinations stated above should be considered only as 10 examples. Other combinations fulfilling the conditions specified and also thecondition of being semiconducting, i.e. having resistivity within the range of10-1-106 ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fal!within the scope of the invention.

The insulating layer may consist, for example, of a solid thermoplastic 15 materiai such as low-density polyethylene (LDPE), hiah-density polyethylene(HDPE), poiypropylene (PP), polybutylene (PB), polymethyl pentane (PMP),cross-iinked materials such as cross-iinked polyethylene (XLPE or PEX), orrubber such as ethylene propylene rubber (EPR) or Silicon rubber.

The inner' and outer semiconducting layers may be of the same basic 20 materiai but with particles of conaucting materiai such as soot or métal powdermixed in.

The mechanical properties of these materials, particularly their coefficientsof thermal expansion, are affected relatively Utile by whether soot or métal powderis. mixed in or not - ai least in the proportions required to achieve the conductivity 25 necessary according to the invention. The insulating layer and thé semiconductinglayers thus hâve sucstantially the same coefficients of thermal expansion.

Ethyiene-vinyl-acetate copciymer/nitriie rubber, butylymp polyethylene.ethylene-acrylate-ccpoiymers and et'nyiene-ethyi-acryiate copolymers may alsoconstitute suitable polymers for the semiconducting layers. 30 Even v/hen different types of matériel are usad as base in the various layers, ii is desiraole for their coeniciems or thermal expansion to be of the same 011363 order of magnitude. This is the case with the combination of the materials Iistedabove.

The materials Iisted above hâve relatively good elasticity, with an E-modulus of E<500 MPa, preferably <200 MPa. The elasticity is sufficient for any 5 minor différences between the coefficients of thermal expansion for the materialsin the layers to be absorbed in the radial direction of the elasticity so that nocracks or other damages appear and so that the layers are not released fromeach other. The material in the layers is elastic, and the adhesion between thelayers is at least of the same magnitude as in the weakest of the materials. 10 The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outersemiconducting layer is sufficiently large to contain the electrical field in the cable,but at the same time sufficiently small not to give rise to significant losses due tocurrents induced in the longitudinal direction of the layer. Λ 15 Thus, each of the two semiconducting layers essentially constitutes one equipotential surface, and the winding with tnese layers will substantially enclosethe electrical field within it.

There is, of course, nothing to prevent one or more additionalsemiconducting layers being arranged in the insulating layer. 20 According to advantageous embodiments of the machine in accordance with the invention an excitation System for suppiying the field circuit comprises apart rotating with the field circuit, and parts of the detecting circuit for earth faultsare arranged in said rotating part. The detecting circuit comprises a rotatinginjection circuit for application on a measuring circuit that is closed through the 25 impédance between field winding and earth, an injection voltage and a measuringunit for measuring the error current resulting in said measuring circuit from theinjection voltage, rectifier units being arranged to form rectified absolute values ofthe injection voltage and the error current, a wireiess communication unit alsobeing provided to transmit said absolute values to a stationary calculating unit for 30 monitoring the résistance of the field winding to earth. This means that oniy twoprocess signais, namely the rectified absolute values for the injection voltage andthe error current, need be transmitted to the stationary part to détermine the résistance value to earth. This results in a iimited signal interface between thestationary and the rotating part, with less demand on the slip ring-freetransmission. The number of rotating units for injection and measuring is alsoIimited. The calcuiating unit suitably comprises a computer equipment for 5 implementing requisite calculation algorithms.

According to another advantageous embodiment ci the machine in accordance with the invention, in which the excitation System is supplied from anexciter with rotating stator side, the injection circuit is supplied from the rotatingstator side of the exciter. Voltage fluctuations can then be ccmpensated for by 1Ü means of software functions in the computer equipment. These functions arebased on known circumstances relating to phase shifting in RC circuits andcalculation of both real and imaginary carrent components and absolute values forlimit value détermination.

According to yet another advantageous embodiment of the machine in 15 accordance with the invention filter circuits are arranged in said measuring circuitin order to filter away harmonies and to block direct voltages. The filter timeconstants for filterina harmonies shall in that case correspond to the period time ofthe injection voltage in order to enable the harmonies to be effectively fiitered off.

According to yet another advantageous embodiment of the machine in 20 accordance with the invention scaling units are arranged prior to a comparator fercomparison of said absolute values of the error current with predeterminea limitvalues, which scaling units are arranged to normalise ar.c compensais themeasured error current for variations in the injection voltage before the errorcurrent is supplied to the compararor. This is of significance since the injection 25 voltage is altered with the excitation.

According to another advantageous embodiment cf the machine in accordance with the invention the above-mentioned problem is soived by theinjection circuit being supplied from a constant voltage source.

According to yet another advantageous embodiment cf the machine in30 accordance with the invention a stationary voltage source'is arrangée to suppiythe injection circuit via a ring transformer. This enables earth faults to be detected even when the rotor is stationary. 6 011363

Brief description of the drawinas

To further explain the invention, embodiments of the invention selected by 'way of example will be describeci in more detail with reference to theaccompanying drawings in which 5 Figure 1 shows a cross section t'nrough the insulated conductor used forwindings in the machine according to the invention,

Figure 2 shows a diagram of the excitation System with circuit for detectingearth faults in the field circuit and with means for determining therotor température in an embodiment of the rotating electric machine 10 according to the invention,

Figures 3-6 show équivalent circuits for the measuring circuit included in thedetecting circuitfor earth faults, in different error cases, and

Figure 7 illustrâtes an embodiment of a scaling unit for normalising andcompensating the measured signal. 15

Description of preferred embodiments of the invention

Figure 1 shows a cross section through an insulated conductor 11 intended for use in at least one of the windings of the machine in accordance withthe invention. The insulated conductor 11 thus comprises a number of strands 35 20 made of copper (Cu), for instance, and having circular cross section. Thesestrands 35 are arranged in the middle of the insulated conductor 11. Around thestrands 35 is a first semiconducting layer 13. Around the first semiconductinglayer 13 is an insulating layer 37, e.g. XPLE insulation. Around the insulating laver37 is a second semiconducting layer 15. The insulated conductor is flexible and 25 this property is retained throughout its sen/ice life. Said three layers 13, 37, 15 aresuch that they adhéré to each other even when the insulated conductor is bent.The insulated conductor has a diameter within the interval 20-250 mm and aconducting area within the interval 80-3000 mm2.

Figure 2 shows a circuit diagram of the excitation System in a rotating 30 electric machine with one or more windings of the insulated conductor shown inFigure 1 to enable direct connection to a power network. The excitation System 011363' 7 comprises both a rotsting injection and suppiy circuit 16 and a stationary unit 20for detecting earth faults and for calculating the rotor température.

The excitation System thus comprises a rotating part 1 equipped with arotating exciter G3 which, from the rotating stator side, supplies a diode orthyristor bridge 12 which is connected by its direct current side to the field windina14 of the machine. An injection and measuring circuit 16 is also provided for usewhen detecting earth faults in the field circuit, and measuring means 18 todétermine the field voltage for température calculations. The rotating part 1 alsoincludes a suppiy means 5 to suppiy the electronic equipment of the rotating part. 10 and also with a communication unit 3. A measuring means 25 is also provided formeasuring the field current If. Wireless communication between the rotating part1 and the stationary equipment 20 is achieved with; the aid of ine communicationunit 3 and a stationary communication unit 4.

By means of an injection circuit comprising a transformer 8 for voltage15 adjustment and gaivanic séparation, the measuring circuit is supplied with asuitable voltage U via an injection transformer 9, said voltage thus beingwithdrawn from the AC side of the exciter G3. The measuring circuit includes twoparallel RC branches and is closed tnrough the impédance of the field winding 14to earth. The RC branches serve as current limitation and DC insulation.

20 The current I generated in the measuring circuit by the injection voltage U is sensed by a sensing circuit 22 via a measuring transformer 11 and converted tea corresponding voltage signai wnich is filtered in t'ne fiiter circuit 24 and rectifiéein the rectifier 26. The voltage signai U| obtained on the output of the rectifier 25.thus represents the amplitude value for the fondamental tone o’f the current I in25 the measuring circuit.

The injection voltage U is also filtered and rectified in simiiar manner inthe fiiter circuit 28 and the rectifier 30. a voltage signa! Uu being obtained on theoutput of the rectifier, which represents the amplitude value for the fundamenta:tone of the injection voltage U. 30 The fiiter time constants T for the filters 24, 23 shall correspond to the period time of the injection voltage U and measured current 1 to effectively fiiter offail harmonies. 011363 8

The voltage signais Uu, VJ| are transmitted by the communication units :4 to the stationary part 20 for calculation of the résistance of the field winding 'to earth from these signais in the calcuiating unit 17.

The calcuiating unit 17 thus enables earth fsults in the field winding 145 be monitored, and an alarm is tripped w'nen the résistance of the field winding -to eart'n falls beiow a predetemnined level.

Rj dénotés the résistance of the field winding 14 to earth. i.e. in practicthe résistance. to the iron mass of the rotating part, and Cj dénotés tfcapacitance of the winding 14 to earth. The résistance Rj may in principle va10 from infinitely large to zéro.

Figure 3 illustrâtes an équivalent circuit for the measuring circuit if Rj = i.e. the "worst" case with the field winding 14 short-crcuited to earth. The résultacurrent 11 in the circuit csn be calculated using known values for the résistancecapacitance C and injection voltage U, and suitable normalising constants car, l15 determined in accordance with principles described in conjunction with Figurebelow. The absolute value of the current 11 corresponds to the value of tlmeasured signal U1 that is transmitted to the calcuiating unit 17, as describ-above in conjunction with Figure 2.

The diagran to the right of the équivalent circuit in Figure 3 illustra;20 magnitudes and phase positions of the injection voltage U. composed ofrésistive comportent Ur and a capacitive component Uc. and the current 11.

Figure 4 shows a correspondinc équivalent circuit in fault-free State, ithe contact résistance to earth is Rj = «. The capsclsnce Ci of ine winding 14earth can be determined using knov/π values for the injection voltage25 résistance R and capacitance C and measuring the current 12.

As in Figure 3, the diaçram to the right of the circuit shows magnitucand phase positions of the injection voltage U, composed of a resistcomponent Ur in phase with the current 12, and a capacitive component consistof the voltage drop U- over the capacitors C and the voltage drop Uj over30 capacitance Cj, and the current 12. 9 011 363

Figure 5 shows a corresponding équivalent circuit in the event of a contactrésistance between winding 14 and earth Rj, where 0<Rj<«, i.e. a State betweenthe States illustrated in Figures 3 and 4. Different limit values for the current I3 foralarm and tripping can, as mentioned in conjonction with Figure 2, be calculated 5 using known values for the résistances R, capacitances C, earthing capacitanceCj, injection voltage U, and the currents 11 and I2 from the cases shown in Figures3 and 4, as well as predetermined limit values for the contact résistance to earthR|-

The impédance Z1 across the two parallel branches, each containing 2R10 in sériés with 2C, is thus 1 Z1=R-J _

wC 15 and the transition impédance between the winding 14 and earth Z2Rj Z2 = _ 1+JwRjCj 20 the current 13 being obtained from I3 = U/(Z1 e Z2)

The diagram to the righi of the circuit in Figure 5 illustrâtes magnncoe;25 and phase positions of voltages and currents in a corresponding manner as

Figures 3 and 4. From this diagram. it is ciear that the current I3 is in phasethe current I2 in Figure 4 and inciudes a current component Iq; througn metransition capacitance Cj anc a current component lu tnrough tne ccms-resistance Rj. the (amer two current components being at right anales to e=c~ 20· ’otiner in the diagram. i.e. phase-shifted 90°.

Figures 3 and 5 shows cases with errors on the DC side of the suppm m the field winding from the exciter G3. see Figure 2. Figure 6 illustrâtes a situaue"with faulis on the AC side of the rectifier bridge 12. A fau't on the AC side m.characterized by the addition of an extra supply source U£C, and by the absence 10 011363 value of the current being composea of two components - one driven by theordinary injection voltage U and one driven by the potential level of the fault pointto earth, represented by the voltage Uac· In the event of faults on the AC side,therefore, the total absolute value of the error current will exceed the limit values 5 calculated in the case illustrated in Figure 5 - often by a good margin - resulting inthe alarm being tripped.

The corresponding phase diagram to the righ.t in Figure 6 corresponds tothat in Figure 5.

In the event of variations in the injection voltage U the measured signais 10 must be compensated by scaling. Alternative^, the predetermined limit values foralarm tripping or releasing, etc. in a comparator must be changed, which isconsiderably more complicated.

Figure 7 shows a scaling unit 32, 34 included in the calculating unit 17 inFigure 2. In this scaling unit 32, 34 the measured value U|, representing the 15 absolute value of the current I, is normalised by multiplying it by a normalisingconstant K1. A suitable magnitude for the normalising constant K1 can bedetermined by means of a measuring procedure in accordance with Figure 3.Similarly, the measured signal Uu for variations in the injection voltage U iscompensated by scaling with a compensation constant K2, wnerein K2=Uu at me 20 time of normalising the measured signal U'. Tne current ln. normalised an::comoensateo with regard to variations in the injection voltage U. is suppliée m acomparator 33 in which this current ln is comparée with various predetermmeeΙίίΓι·: values Lim 1. bm 2. bm 3 for tripping tne alarm. emilting a tripping signe -C i he measuring means 18 measure tne neld voltaoe and the measurme means 25 measures tne üe-c current. and corresponding measured signais b-and If srs iransmitred via the wireless communication units 3, 4 ic a unit 40 in mestationary eguipment 20 for calculating the rotor température from thesemeasured signais, see Figure 2. In the filter 42 in tne measurino means 13 tne 30 field voltage signal is filtered with a time constant T1 which shall correspond te 0.3fîmes the no-!oad time constant of tne field winding 14. When the eiectric machine 011363 is not synchronized on the network, it has a lime constant corresponaina to the no-load time constant, whereas if the machine is connected to the network this time constant is aitered by a factor of approximateiy 0.3, depenaina on the inductance of the network. 5 The unit 40 may in turn be connected to indicating means for the rotor température or alarm, for instance, or tripping means to activate these depenainaon the determined value for the rotor température.

Numerous modifications and variations of the embodiments aescribedabove are of course possible within the scope of the invention. The invention is10 thus also applicable to stationary solutions such as static exciters, and the supplyvoltage to the injection unit can be transformed to the rotating part by means of aring transformer so that earth faults can also be detected wnen the machine isstationary.

Claims (19)

12 η 1 1 7 r 7υ ί ι u u Ο CLAIMS

1. A rotating electric machine of a type with rotating field circuit, whichmachine is intended for direct connection to a distribution or transmission network, 5 characterized in that at least one electric winding of the machine comprises atleast one electric conductor, a fîrst layer with semiconducting propertiessurrounding the conductor, a soiid insulating layer surrounding the fîrst layer, and .a second layer with semiconducting properties surrounding the insulating layer,and in that a detecting circuit is provided for detecting earth faults in the rotating 10 field circuit.

2. A machine as claimed in claim 1, characterized in that the potentiâl of thefirst layer is substantially similar to the potentiâl of the conductor. 15 3. A machine as claimed in claim 1 or claim 2, characterized in that the second layer is arranged to form a substantially equipotential surfacesurrounding the conductor.

4. A machine as claimed in claim 3, characterized in that the second layer 20 is connected to a predetermined potentiâl.

5. A machine as claimed in claim 4, characterized in that saidpredetermined potentiâl is èartn potentiâl. 25 6. A machine as claimed in any of the preceding daims, characterized in that at least two adjacent layers of the machine winding hâve substantially thesame coefficients of thermal expansion.

7. A machine as claimed in any of the preceding claims, characterized in 30 that the conductor comprises a number of strands, at least some of which are inelectrical contact with each other. 011363 8.' A machine as claimed in any of the preceding daims, çharacterized inthat each of said three layers is firmly joinea to adjacent layers along substantiallyits whole contact surface. 5 9. A machine as claimed in any of the preceding daims, characterized in t'nat said layers are arranged to adhéré to each other even when the insulatedconductor is bent.

10. A rotating electric machine of a type with. rotating field circuit, which 10 machine is intenaed for direct connection to a distribution or transmission network,characterized in that at least one winding of the machine is formed of a cablecomprising one or more current carrying conductors, each conductor having anumber of strands, an inner semiconducting layer arrangea around eachconductor, an insulating layer of solia insulating material arranged around said 15 inner semiconducting layer, and an outer semiconducting layer arranged aroundthe insulating layer, and in that a detecting circuit is arranged to aetect eartn faultsin the rotating field circuit.

11. A machine as ciaimed in ciaim 10, characterized in that said cable 20 comprises a sheath.

12. A machine as claimed in any of the preceding ciaims, characterized inthat an excitation System for supplying the field circuit comprises a pan. rotatingwith the field circuit, and in that an injection and measuring unit for said detecting 25 circuit is arranged in said rotating part. 13À A machine as claimed in any of the preceding ciaims. characterized inthat the detecting circuit comprises an injection circuit for application on ameasuring circuit that is closed through the impédance between field winding and 50 earth, an injection voltage and a measuring unit for measuring the error currentresulting in said measuring circuit from the injection voltage, and in that rectifierunits are arranged to form rectified absolute values of the injection voltage and the 0.11363 14 error current, a wireless communication unit also being provided to transmit saidabsolute values to a stationary calculating unit for monitoring the résistance of thefield winding to earth. 5 14. A machine as ciaimed in claim 13, wherein the excitation System is supplied from an exciter with rotating stator side, characterized in that theinjection circuit is supplied from the rotating stator side of the exciter.

15. A machine as ciaimed in claim 13 or claim 14, characterized in that filter 10 circuits are arranged in said measuring circuit in order to filter away harmonies and to block direct voltages.

16. A machine as ciaimed in any of ciaims 13-15, characterized in that acomparator is arranged to compare said absolute values of the error current with 15 predetermined limit values and, depending on the resuit of the comparison, to tripalarms.

17. A machine as ciaimed in claim 16, characterized in that scaling units arearranged prior to the comparator in order to normalise and compensate the 20 measured error current for variations in the injection voltage before the errorcurrent is supplied to the comparator.

18. A machine as ciaimed in any of the preceding ciaims, characterized inthat measuring means are arranged to measure the voltage and current of the 25 field winding and transmit these values to a unit for calculating the rotor température. !

19. A machine as ciaimed in claim 18, characterized in that the unit forcalculating the rotor température is stationary and in that said measured voltage 30 and current values for the field winding can be transmitted to said calculating unitvia the wireless communication unit. i5 01 1 363

20. A machine as claimed in claim 18 or claim 19, characterized in that analarm is connected to the calculating unit which alarm is tripped when thetempérature exceeds a predetermined limit value. 5 21. A machine as claimed in claim 13, characterized in that a stationary voltage source is arranged to supply the injection circuit via a ring transformer.

22. A machine as claimed in claim 13, characterized in that the injectioncircuit is supplied from a constant voltage source. . 10

23. A method for a rotating electric machine of a type with rotating field circuit,which machine is intenaed for direct connection to a distribution or transmissionnetwork, wherein ai least one electric winding of the machine comprises at leastone electric conducior, a first layer with semiconducting properties surrounding 15 the conductor, a solid insulating layer surrounding the first layer, and a secondlayer with semiconducting properties surrounding the insulating layer,characterized in that an injection voltage is supplied to a measuring circuit that isciosed through the impédance between field winding and earth, and the resultingerror current in the measuring circuit is measured, whereupon rectified absoiute 20 values of the injection voltage and the error current are formed and transmitted toa calculating unit for monitorina the résistance of the field winding to earth.

24. A method as claimed in claim 23, characterized in that harmonies in themeasuring circuit are filtered away.

25. A method as claimeo in 23 or claim 24, characterized in that saidsbsolute values or the error current are compared with predetermined limit valuesand an alarm is tripped aependinc on the resuit of the compariscn. 30 26. A method as claimed in claim 25, characterized in that prior to the comparison, the error current measured is normalised and compensated forvariations in the injectina voltage. 16 011363

27. A method for a rotating electric machine of a type with ratatina field circuit,which machine is intended for direct connection to a distribution or transmissionnetwork, wherein at least one electric winding of the machine comprises at least5 one electric conductor, a first layer with semiconducting properties surroundingthe conductor. a solid insulating layer surrounding the first layer, and a secondlayer with semiconducting properties surrounding the insulating layer,characterized in that the voltage and current of the field winding are measuredand the rotor température is calculated from these measured values. 10